Magnetic recording media are widely employed as audio tapes, video tapes, computer data tapes, floppy disks, and the like. Recording media of this type basically comprise a non-magnetic support having provided thereon a magnetic layer comprising a binder having dispersed therein a ferromagnetic powder.
Magnetic recording media are essentially required to have high levels of performance properties, such as electromagnetic characteristics, running durability, and running properties. In particular, through the recent spread of 8 mm video tape recorders, there is an increasing demand for video tapes to have high video outputs, excellent image reproducibility, and especially excellent electromagnetic characteristics.
Various improvements have been proposed with respect to electromagnetic characteristics of the magnetic recording media. Inter alia, introduction of an improvement in a ferromagnetic powder, i.e., a magnetic recording substance, is a direct and effective approach for achieving improved electromagnetic characteristics. Accordingly, there are tendencies that ferromagnetic powders are made finer and that iron oxide as a ferromagnetic powder has been replaced by iron oxide doped with different metals, e.g., cobalt. Quite recently, use of ferromagnetic metals, e.g., iron, nickel and cobalt, or alloys containing them has been extending.
Use of these improved ferromagnetic powders should reasonable make it possible to provide magnetic recording media having satisfactory electromagnetic characteristics. Actually, however, it is difficult to produce magnetic recording media manifesting improved electromagnetic characteristics in conformity with the improved characteristics of the ferromagnetic powder used. This is because dispersibility of ferromagnetic powders tends to be reduced as the powders become finer.
Moreover, there is a tendency that dispersibility of ferromagnetic powders decreases in the order of .gamma.-Fe.sub.2 O.sub.3, Co-doped .gamma.-Fe.sub.2 O.sub.3, and a ferromagnetic metal fine powder. That is, some improvement on ferromagnetic powders may result in deterioration of dispersibility of the powder, thereby failing to enable full use of the excellent characteristics of the improved ferromagnetic powder.
In order to obtain improved dispersibility of a ferromagnetic powder, it has been proposed to use binders containing a polar group, e.g., --SO.sub.3 M, --OSO.sub.3 M.sub.1, --PO.sub.3 M.sub.2, --OPO.sub.3 M.sub.2, and --COOM (wherein M is a hydrogen atom, an alkali metal, or an ammonium ion), or binders containing such a polar group and an epoxy ring as disclosed in JP-B-58-41565 (the term "JP-B" as used herein means an "examined published Japanese patent application"), JP-A-57-44227, JP-A-59-30235, JP-A-60-238306, JP-A-60-238309, and JP-A-60-238371 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). These binders exhibit high adsorptivity onto ferromagnetic powders and thereby improved capability of dispersing the powders as compared with conventional binders. Nevertheless, as the particle size of ferromagnetic powders is reduced for realizing high-density recording, dispersing becomes difficult even with a binder selected from these improved ones. It has, hence, been necessary to develop techniques to take full advantage of these binders having high dispersing ability.
It has been suggested to improve dispersed state of a ferromagnetic fine powder by conducting kneading and dispersing for preparing a magnetic coating composition for an extended period of time. However, since a considerable shearing force is exerted on the ferromagnetic fine powder during kneading and dispersing, characteristics of the powder are sometimes impaired. In addition, requirement of a long time for the production of a magnetic recording medium is undesirable for working efficiency.
It has also been proposed to conduct kneading by using a specific binder, e.g., a water-soluble carboxylic acid resin (see JP-B-57-42888) and polyvinyl butyral (see J-A-59-165237), and then add other binders or solvents to the mixture, followed by further dispersing, but sufficient dispersing effects have not yet been obtained.
Studies have hence been directed to techniques for effectively dispersing the above-described ferromagnetic powder without greatly alterating the process for producing general magnetic recording media. Methods so far proposed to this effect include a method of subjecting a ferromagnetic fine powder to surface treatment with a surface treating agent, e.g., a silane coupling agent, a method of using an additive for improving dispersibility of a ferromagnetic powder, e.g., fatty acids, and a method of using a binder system comprising a vinyl copolymer and a urethane prepolymer having incorporated thereinto a low-molecular epoxy resin (see JP-B-56-23210).
The treatment with a silane coupling agent makes the surface of the ferromagnetic powder hydrophobic to generally improve dispersion stability of the powder in a magnetic coating composition. In turn, however, the silane coupling agent-treated powder sometimes has reduced compatibility with resinous components and, in such cases, the final dispersed state of the ferromagnetic powder in a magnetic layer is not sufficient. Besides, the silane coupling agent itself is too expensive as a treating agent for ferromagnetic powders to be used in average magnetic recording media.
Fatty acids are usually used as a lubricant in a magnetic layer of magnetic recording media. Since fatty acids also have an effect of dispersing a ferromagnetic powder, it is possible to improve the dispersion of a ferromagnetic powder by controlling the amount of the fatty acid to be added. In general, however, sufficient dispersing effects of the fatty acids cannot be produced unless they are added in amounts larger than necessary for use as a lubricant for magnetic layers. It is known that a fatty acid when used in excess acts as a plasticizer of binders. Accordingly, use of the fatty acid as a dispersing agent for a ferromagnetic powder necessarily gives rise to the problem of plasticization of the binder.
On the other hand, improvement of dispersion of a ferromagnetic powder leads to smoothing of the surface of a magnetic recording medium, which results in deterioration of running properties. For example, addition of a large amount of a fatty acid brings about an improvement of dispersion but is insufficient in accomplishment of improvements in durability or running properties.
Further, when a low-molecular epoxy resin is incorporated into the specific binder system, with which a ferromagnetic powder is kneaded, there are disadvantages in that the kind of the binder is limited, the improvement on dispersibility of the ferromagnetic powder is not sufficient, and an extended kneading time impairs characteristics of the ferromagnetic powder.
JP-A-63-146218 also proposes use of a low-molecular epoxy compound as a binder, but it is used as one of the components of a binder system similar to JP-B-56-23210.
However, according to the techniques using low-molecular epoxy compounds, kneading or dispersing of the ferromagnetic powder is carried out by using a large amount of a binder or a solvent so that the mixture cannot be vigorously kneaded.
It has been proposed to use a vinyl chloride-based copolymer (e.g., a vinyl chloride-vinyl acetate-maleic anhydride copolymer) in combination with a polyurethane resin as binder resins for a ferromagnetic metal fine powder. The combined use of these binder resins makes it possible to form a magnetic layer having improved characteristics required for a magnetic recording medium, such as strength.
According to the present inventors' examination, the vinyl chloride copolymer used as a binder resin sometimes releases chlorine bonded thereto in the form of hydrogen chloride when the magnetic recording medium is left to stand under severe conditions, for example, at a high temperature (e.g., above 80.degree. C.) and a high humidity, for a long time. The released hydrogen chloride can corrode the ferromagnetic metal fine powder and ultimately other metallic parts of a reproducing device such as a video tape deck, e.g., a supporting rod, a Perrmalloy head, etc.
The inventors have developed a magnetic recording medium having a magnetic layer comprising a vinyl chloride-based copolymer and a polyurethane resin as binder resins, said vinyl chloride copolymer being stabilized by introducing a specific polar group and an epoxy group, as disclosed in JP-A-61-253627. Because the vinyl chloride copolymer resin therein used is stable, the proposed magnetic recording medium, even when placed in long-term contact with a reproducing device, such as a video tape deck, under, e.g., a high temperature and high humidity condition, causes virtually no corrosion of metallic parts of the device. Neither does it cause corrosion of a ferromagnetic metal fine powder, when used.
As a result of further investigations on the above-described magnetic recording medium, however, the inventor have found that even such a specific binder system tends to encounter difficulty in dispersing a ferromagnetic powder in special cases, for example in using a highly pulverized ferromagnetic powder. In other words, despite the use of a finely pulverized ferromagnetic powder, the resulting magnetic recording medium fails to manifest sufficiently improved electromagnetic characteristics in conformity with the improvement of the ferromagnetic powder itself.
In order to cope with the recent demand for high-density recording, there is a tendency that the surface of a magnetic layer should be smoothened as much as possible to thereby minimize spacing loss with a magnetic head. It is known that smoothing of the magnetic layer increases the coefficient of friction on contact with a magnetic head, a cylinder, a guide roller, etc. to thereby deteriorate running properties and durability. Use of various lubricants has therefore been proposed. Examples of proposed lubricants typically include fatty acids as disclosed in JP-B-44-18221 and JP-B-48-15007, fatty acid esters as disclosed in JP-B-43-23889 and JP-B-41-18065, and fatty acid amides as disclosed in JP-B-41-16984, JP-B-47-15624, JP-A-50-136009, JP-A-55-139637, and JP-A-54-46950 and, in addition, other fatty acids, silicone oils, fatty acid-modified silicones, and so on. Among them, fatty acids are excellent in lubricating properties as markedly reducing coefficient of friction and improving still mode durability and are thus frequently employed in combination with fatty acid esters or fatty acid-modified silicones. In this connection, resins having a polar group, e.g., a sulfonic acid radical and a phosphoric acid radical, and an epoxy group (see JP-A-61-123017 and JP-A-61-253627) have conventionally been used in combination with fatty acids or fatty acid esters.
Nevertheless, the above-stated combinations of binders and lubricants has not produced as great an effect on lubricating properties and dispersing properties as expected. It has hence been necessary to develop improved binders and lubricants suitable for use of the ultrafine ferromagnetic powders.